ASTM C39/C39M
Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens
The compressive strength test — the 'psi test' that determines whether delivered concrete met its specified strength.
Who needs it
Ready-mix suppliers, testing labs, inspectors, and any contractor whose spec requires strength verification.
What the test involves
A molded cylinder of concrete is capped so its ends are flat and parallel, then loaded in a calibrated compression machine until it fractures. The machine records the maximum load, which is divided by the cylinder's cross-sectional area to give a strength in pounds per square inch. The method also defines how fast the load may be applied and how the fracture pattern should be reported, because both affect the number.
Common misconceptions
- A single low cylinder does not mean the concrete failed. Acceptance is judged on averages of multiple tests under the governing building code, not on one break.
- The test measures the cylinder, not the slab. A cylinder that was mishandled, left in the sun, or cured in a job trailer can break low even when the in-place concrete is fine.
- Breaking cylinders early does not predict the final number reliably. Early-age breaks are an indicator for stripping forms or opening to traffic, not a substitute for the specified test age.
Official source
We summarize scope and purpose only. The full text belongs to the issuing body — read it at the official source. ASTM International